An injection molding installation is frequently provided with a worm or screw for plasticizing the synthetic resin material and for displacing it axially in a cylinder to force the plastified material into the mold through an outlet at the end of this cylinder (see, for example, German open application No. 28 36 165, or German utility model GM No. 76 39 875).
It is known, for example, from the above identified German application, to provide the worm or screw with a backflow preventer or checkvalve which is designed to prevent, as the injection pressure builds up ahead of the screw, the flowable synthetic resin from reversing its direction and returning into the plastifying compartment.
It has been found to be advantageous to make this valve adjustable with respect to the response time to compensate for the differences in the viscosity or other rheological properties of the synthetic resin material. It is also important that the checkvalve open only at a certain pressure to insure effective plastification and homogenization of the molten synthetic resin material.
While adjustable checkvalves have been provided in the past, they have been comparatively expensive, complicated to adjust and unreliable. The most important disadvantage, however, was the difficulty which was encountered when resetting of the checkvalve for materials with other rheological properties than those previously used was necessary.